CSS Conditional Rules Module Level 4

Editor’s Draft,

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L. David Baron (Mozilla)
Elika J. Etemad / fantasai (Apple)
Chris Lilley (W3C)
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This module contains the features of CSS for conditional processing of parts of style sheets, based on capabilities of the processor or the environment the style sheet is being applied in. It includes and extends the functionality of CSS Conditional 3 [css-conditional-3], adding the ability to query support for particular selectors [SELECTORS-4] through the new selector() notation for supports queries.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-conditional” in the title, like this: “[css-conditional] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 03 November 2023 W3C Process Document.

1. Introduction

The features in level 3 are still defined in [css-conditional-3] and have not yet been copied here.

This level adds extensions to the @supports rule to allow testing for supported selectors.

2. Extensions to the @supports rule

This level of the specification extends the <supports-feature> syntax as follows:

<supports-feature> = <supports-selector-fn> | <supports-decl>
<supports-selector-fn> = selector( <complex-selector> )

The result is true if the UA supports the selector provided as an argument to the function.

This example tests whether the column combinator (||) is supported in selectors, and if so uses it to style particular cells in a table.
@supports selector(col || td) {
  col.selected || td {
    background: tan;

Any namespace prefixes used in a conditional group rule must have been declared, otherwise they are invalid [css-conditional-3]. This includes namespace prefixes inside the selector function.

This example tries to check that attribute selectors with CSS qualified names are supported, but is invalid, because the namespace prefix has not been declared.
@supports selector(a[xlink|href]) {
  // do something, but fail
This example checks that attribute selectors with CSS qualified names are supported.
@namespace x url(http://www.w3.org/1999/xlink);
@supports selector(a[x|href]) {
  // do something

2.1. Extensions to the definition of support

A CSS processor is considered to support a CSS selector if it accepts that all aspects of that selector, recursively, (rather than considering any of its syntax to be unknown or invalid) and that selector doesn’t contain unknown -webkit- pseudo-elements.

Note: Some functional selectors are parsed forgivingly, i.e. if some arguments are unknown/invalid, the selector itself is not invalidated. These are nonetheless unsupported

Security Considerations

No Security issues have been raised against this document

Privacy Considerations

The selector() function may provide information about the user’s software such as its version and whether it is running with non-default settings that enable or disable certain features.

This information can also be determined through other APIs. However, the features in this specification are one of the ways this information is exposed on the Web.

This information can also, in aggregate, be used to improve the accuracy of fingerprinting of the user.


The editors would like to thank all of the contributors to the previous level of this module.


Changes since the Candidate Recommendation Snapshot of 17 February 2022

Changes since the First Public Working Draft of 3 March 2020

Additions since Level 3


Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.


Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.


Terms defined by this specification

Terms defined by reference


Normative References

David Baron; Elika Etemad; Chris Lilley. CSS Conditional Rules Module Level 3. URL: https://drafts.csswg.org/css-conditional-3/
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. URL: https://drafts.csswg.org/css-values-4/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. URL: https://drafts.csswg.org/selectors/

Informative References

Elika Etemad. CSS Namespaces Module Level 3. URL: https://drafts.csswg.org/css-namespaces/

Issues Index

The features in level 3 are still defined in [css-conditional-3] and have not yet been copied here.


In all current engines.

Edge (Legacy)12+IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile12.1+